1. Field of the Invention
The invention relates to a vortex flow meter with a measurement tube through which a medium can flow, with at least one baffle body for generating vortices in the medium and with at least one deflection body which can be deflected at least following pressure fluctuations which accompany the vortices in the medium.
2. Description of Related Art
The measurement principle of vortex flow meters is based on the fact that a so-called Karman vortex street can arise in a liquid or gaseous medium behind a baffle body around which the medium flows and is formed by a vortex which is detached from the baffle body and which advances with the flow. The frequency with which the vortices detach from the baffle body is dependent on the flow velocity, this relationship being almost linear under certain assumptions. Via measurement of the vortex frequency, therefore, the flow velocity of the medium can be determined, as a result of which the volumetric flow rate and mass flow rate can be determined with additional consideration of the pressure and temperature of the medium, for example.
For measuring the vortex frequency, a deflection body has been preferably used in the prior art. The vortices of the medium which occur in the vortex street lead to local pressure fluctuations which act on the deflection body and are detected by it. The deflection body can be a pressure transducer which is implemented, for example, with piezoelements or can be capacitive pressure sensors in which a sensor element is deflected, even if to a small extent. It is important simply that the deflection body is located in the vortex street such that the vortices produced by the baffle body at least indirectly drift past the deflection body, and thus, can be detected. To do this, the deflection body can be provided downstream behind the baffle body. In this case, the baffle body and the deflection body are made physically separate. However, the deflection body can also be the baffle body itself or can be implemented in the baffle body if, for example, in a design known from the prior art, with pressure transducers, the pressure transducers being located over or in the baffle body and the pressure fluctuations of the vortex street indirectly detected via channels in this way; in this case, the baffle bodies and deflection bodies are physically implemented in one unit.
In the methods known from the prior art for detecting the motion of the deflection body in which capacitive or inductive effects are used and in which piezoceramics are used or in which optical fibers are used for detection of the deflection, the deflection bodies must make contact with electric or optical lines. These lines in turn must be routed out of the medium-filled space through the measurement tube wall or the housing of the vortex flow meter into a medium-free space, conventionally, to evaluation electronics. The accompanying penetrations require very complex seals since, depending on the application, high pressure and/or temperature stability must be achieved (several 100 bar, several 100° C.).
Depending on the process conditions, for example, high temperatures of the measurement medium, it can also be that many types of sensors cannot be used. Thus, for example, in the case in which the temperatures of the medium are above the Curie point, piezoceramics cannot be used.
In the prior art, there are still other versions of the sensor technology for measuring the vortex frequency or the pressure fluctuations which accompany the vortices.
U.S. Pat. No. 3,823,610 discloses a vortex flow meter in which the motion of a ball as a result of pressure fluctuations is evaluated for measuring the vortex frequency.
The teaching of U.S. Pat. No. 4,181,020 consists in the proposal to provide a pivotally mounted flag, which moves as a result of pressure fluctuations, behind the baffle body. The structure in International Patent Application Publication WO 92/01208 A 1 is similar; in it, the movements of a deflection body are optically measured outside the measurement tube.
U.S. Pat. No. 7,770,469 B2 describes a differential pressure sensor for measuring the pressure fluctuations which occur as a result of vortices. In the measurement set-up, there is a microwave resonator with an elastic membrane, the membrane reacting to the pressure fluctuations such that the resonant frequency of the resonator changes.
In order to avoid the problem that the flow direction is dictated by the arrangement of the deflection body and the baffle body, U.S. Pat. No. 4,735,094 discloses an arrangement in which there is a sensor for measurement of the vortex frequency between two baffle bodies along a longitudinal axis of the measurement tube.
To increase the measurement accuracy, U.S. Pat. No. 4,831,883 proposes placing two baffle bodies with two sensors next to one another.